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"other" Fender mods

12/5/1999 7:15 AM
woofmain	"other" Fender mods Are there alternate methods of incrementally/gently pushing up the gain/drive in Fender amps - i.e., changing input jack resistor values, slightly higher value plate load resistor values, slightly lower cathode resistor values etc. ? What are plus and minus aspects of these approaches ? Woofmain >

12/5/1999 9:06 AM
Carl Z	Hi woofmain; 1) input jack resistor values: The 68k/1 meg setup is probably about as good as you're going to get here. 2) Plate load resistors: Changing the plate load from say 100k to 220k is going to reduce the tube current somewhat and give you a little more negative bias. This is actually going to make the stage run cleaner but the net gain will increase. This can allow you to hit the driver a little harder. 3) Cathode resistors: Decreasing the size of the cathode resistor is going to give you more current through the tube and thus a little less headroom. Because this resistor is typically bypassed with a cap you won't see any substantial gain increase in the stage. You may also run the risk of having the amp fart out on you. 4) Other suggestions: One easier way to get a whopping gain increase is to move the tone stack from after the first gain stage to after the second stage. This is pretty easy to do in a Fender by just swapping the stages that the volume pot goes to. For a few ideas along these lines go to Steve A's site, http://www.techaccessinc.com/blueguitar/ and check out the schematic of the Blues Junior. You can also try knocking back the grid resistors to the power tubes from 220k to about 100k. This has several effects. First, it changes the time constant of the RC network and gives you a little faster response. Second, the gain of the driver will be reduced letting you drive the driver a little harder( sheesh, now that's reduntant for you!) before you start to clip the power tubes. Last suggestion; play around with the feedback network a little. I emphasize the word little here! You can easily turn the power amp into an oscillating nightmare if you get too wild with the changes. I'd suggest starting by changing the 820 ohm feedback resistor to something in the neighborhood of 33k. Temporarily wiring in a 50k pot as a rheostat here until you find a value that you like is a lot easier than swapping resistors in and out. Hope these suggestions point you in the right direction. Regards; Carl Z Summit Amps http://www.geocities.com/summitamps

12/5/1999 9:06 AM

Carl Z

Hi woofmain;

1) input jack resistor values: The 68k/1 meg setup is probably about as good as you're going to get here.

2) Plate load resistors: Changing the plate load from say 100k to 220k is going to reduce the tube current somewhat and give you a little more negative bias. This is actually going to make the stage run cleaner but the net gain will increase. This can allow you to hit the driver a little harder.

3) Cathode resistors: Decreasing the size of the cathode resistor is going to give you more current through the tube and thus a little less headroom. Because this resistor is typically bypassed with a cap you won't see any substantial gain increase in the stage. You may also run the risk of having the amp fart out on you.

4) Other suggestions:
One easier way to get a whopping gain increase is to move the tone stack from after the first gain stage to after the second stage. This is pretty easy to do in a Fender by just swapping the stages that the volume pot goes to. For a few ideas along these lines go to Steve A's site, http://www.techaccessinc.com/blueguitar/ and check out the schematic of the Blues Junior.

You can also try knocking back the grid resistors to the power tubes from 220k to about 100k. This has several effects. First, it changes the time constant of the RC network and gives you a little faster response. Second, the gain of the driver will be reduced letting you drive the driver a little harder( sheesh, now that's reduntant for you!) before you start to clip the power tubes.

Last suggestion; play around with the feedback network a little. I emphasize the word little here! You can easily turn the power amp into an oscillating nightmare if you get too wild with the changes. I'd suggest starting by changing the 820 ohm feedback resistor to something in the neighborhood of 33k. Temporarily wiring in a 50k pot as a rheostat here until you find a value that you like is a lot easier than swapping resistors in and out.

Hope these suggestions point you in the right direction.

Regards;
Carl Z
Summit Amps
http://www.geocities.com/summitamps

12/5/1999 6:25 PM

Bruce

Re: "other" Fender mods: My two cents worth...

quote:
"1) input jack resistor values: The 68k/1 meg setup is probably about as good as you're going to get here."

Maybe, but there are some subtle tweaks here that can make a flat guitar pickup sound better or an overly hot pickup work better with farty sounding Fender amps.

quote:
"2) Plate load resistors: Changing the plate load from say 100k to 220k is going to reduce the tube current somewhat and give you a little more negative bias. This is actually going to make the stage run cleaner but the net gain will increase. This can allow you to hit the driver a little harder."

You'll have to explain this one a little better Carl.
I agree with parts but, that isn't "quite" what my experience has been... :>)

quote:
"3) Cathode resistors: Decreasing the size of the cathode resistor is going to give you more current through the tube and thus a little less headroom. Because this resistor is typically bypassed with a cap you won't see any substantial gain increase in the stage. You may also run the risk of having the amp fart out on you."

Too a point, but when the current through the cathode resistor is enough to drop the plate voltage to a value that is bit higher then half of the B+ applied, I think you'll find that is where the tube is actually the sweetest and still has excellent headroom.
I prefer to keep the "standard" 12AX7 tridode ideling around 1ma to 1.2ma using other parameter values like, Ep=200vdc with an Rp of about 91K to 120K, Rk 1K to 1K5 and Ck of about 10uF to 22uF.
Keep in mind, the 12AX7 triodes are normally used as class A amps and need the proper bias, current and plate voltage to keep them running in their linear curve with the max amount of gain desired.

quote:
"4) Other suggestions:
One easier way to get a whopping gain increase is to move the tone stack from after the first gain stage to after the second stage. This is pretty easy to do in a Fender by just swapping the stages that the volume pot goes to. For a few ideas along these lines go to Steve A's site, http://www.techaccessinc.com/blueguitar/ and check out the schematic of the Blues Junior."

True, the typical Fender tone stack, shoved in between the two halves of a 12AX7, does use up quite a bit of signal.
But I think it has the best bang for the buck in terms of overall tonal effect vs cost, parts count and ease of construction, and as long as the next triode stage has good gain... (called the tone recovery stage for a reason)... it works wonderfully right where it is.
There are other places you can get more drive, if you want or need it, other then shifting the tone stack to after the traditional tones stack recovery section.
Maybe Randall A. can add something here too.
I know Randall has a particular interest in low Zed cathode driven tone stacks and the effect they have on the driver.

quote:
"You can also try knocking back the grid resistors to the power tubes from 220k to about 100k. This has several effects. First, it changes the time constant of the RC network and gives you a little faster response. Second, the gain of the driver will be reduced letting you drive the driver a little harder( sheesh, now that's reduntant for you!) before you start to clip the power tubes."

I agree!
This is a good mod for many Fender amps that can benifit from some additional preamp "tone" before the power tube fart out.
But, this is not quite so when using cathode biased power tubes.
And dropping the coupling caps value a notch or so, from say, .1uF tp .047uF to even .033uF, can go a long way in controlling the woofie nasties too.

quote:
"Last suggestion; play around with the feedback network a little. I emphasize the word little here! You can easily turn the power amp into an oscillating nightmare if you get too wild with the changes. I'd suggest starting by changing the 820 ohm feedback resistor to something in the neighborhood of 33k. Temporarily wiring in a 50k pot as a rheostat here until you find a value that you like is a lot easier than swapping resistors in and out."

I agree again!
Changing the feedback loop voltage lower by a slight amount, but staying reasonable, is a great way to get a more "fun" sounding amp.
It will really loosen things up.
But, going from 820ohms to 33K-50K , is too much for me in an otherwise normal good sounding Fender amp.
OK if you play only mournfully bluesy slide guitar...
Did I say woofie?
Use small incremental amounts when doing this or use a small 25k-50k trimpot in place of the 820 ohm resistor on the eyelet board.
You'll HAVE to go back to the PI/driver and shut it down a little or into the preamp stages and reduce their outputs a a bit here and there to get really good results if you try and use somethng as large as 33K to 50K.
Personally, when playing at any decent volume, without redoing the PI/driver, I don't like the sound at all when using anything past about 3K for the FB resistor.
You can also yank off the grounded 47 to 100 ohm resistor
and use a 200ohm to 500ohm 1/2w resistor.
By using a 500ohm to 1K resistor you can bypass it with a .22uF to 1uF cap to get a cranked with presence control sound too.

Good post addressing some solid points Carl!
Anyone else?
Bruce

12/6/1999 2:43 AM
Paul C	Another plug for dropping the pwr tube grid leaks down... I've never liked the sound of the vib kill switch trick, but with the changes to the pwr tube inputs they can deal with the increase in level much smoother. Also these amps sort of have a master volume inside. The ch mix resistors/plate loads set up a divider that dumps a large amount of signal (the ratio isn't the same for each channel because of the vib 50k pot). If killing the vib pot is to large of a boost you can dink with the values of the mix resistors. For a little more gain try a 150k series resistor instead of the 220k. For a little less change the other resistor. Of course this will have the opposite effect on the other channel you're not using!

12/6/1999 2:43 AM

Paul C

Another plug for dropping the pwr tube grid leaks down... I've never liked the sound of the vib kill switch trick, but with the changes to the pwr tube inputs they can deal with the increase in level much smoother. Also these amps sort of have a master volume inside. The ch mix resistors/plate loads set up a divider that dumps a large amount of signal (the ratio isn't the same for each channel because of the vib 50k pot). If killing the vib pot is to large of a boost you can dink with the values of the mix resistors. For a little more gain try a 150k series resistor instead of the 220k. For a little less change the other resistor. Of course this will have the opposite effect on the other channel you're not using!

12/6/1999 3:21 AM
Carl Z	Re: "other" Fender mods: A little long winded Bruce; Lets see if I can tighten these explainations up a bit. First I was running under the assumption that woof was looking for more distortion in his rig. As I'm sure you're aware tone is mighty subjective so what's pleasing to one person may not be pleasing to another. Case in point...I've got a customer that loves his marshall biased like an ice cube. He loves the tone of his EL34's running at about 20mA/tube. To each his own. Let's start with the input jack. Hot pickups such as humbuckers or even active pickups really could use some loading before they hit that first stage so you could reduce the size of that 1 meg resistor. The 68k grid stopper can be twiddled to give you a littl more high end and clarity by reducing the value to something like 33k or 22k. You could even remove it all together. Thumb rule here is that lower values will extend the high frequency response resulting in a crisper tone while larger values tend to dull the tone somewhat. Another tweak here is to run a bypass cap across it like some of the Ampegs. Trace (where'd ya go?) had a mod for this but I never got around to trying it. Plate resistors: Without writing a book I'll try to clarify this a little. I'm ballparking the numbers here so i may be off a little. If we assume we're running with normal channel fender; gain -> tone -> recovery, changing the plate load on the second stage to 220k is going to give you an amplification factor somewhere in the neghborhood of 70 and bias current is going to be about 0.68mA. This gives you a bias voltage of just about 1 volt. I probably shouldn't have used the word headroom here, got sloppy with the terminology. What I meant to say was that the stage is going to run a little closer to pure class A and the THD is going to be much lower. I want to say it's about 1%, feel free to correct me if i'm wrong here. Unless you're running the amp flat out or again using hot pickups you probably won't drive it into distortion. With the volume maxed a strat will give you a little over a volt but by then you'll have power amp clipping. Hope I cleared that one up. On to the tone stack.... I've tried the blues junior arrangement and it's not too shabby, just different. It's a quick easy mod to that can be undone in a couple of minutes if you don't like it. From a purely engineering point of view passive tone controls really need to be placed after a low impedance source such as a CF stage. In this regard I've had some interesting results on a reverb amp by bootstrapping the first two stages and running the tone off the follower. Then tapping the tone control to send to the reverb circuit and back to the mixer stage. Definetly worth further investigation by the adventurous! As for the grid resistor RC network I tend to prefer the 047 caps. 022 and 033 caps seem a little too constricted to me. There's that personal preference thing again. On the feedback loop I like the sound of 22k/4.7k combination. To me it's a nice compromise. You could also bypass the 22k with a small cap of around 250p but I'm not a big fan of that. seems to roll of too many highs. Anyway, to make a long story short here's what I like to do to my Fenders.... 1) change the input grid stoppers to 33k 2) change the cathode bypass caps to 5uF. 3) change the power tube grid resistors to 100k and the grid stoppers to 4.7k 4) change the driver coupling caps to 0.047uF 5) change the driver input cap to 0.01uF 6) change the feedback loop values to 22k/4.7k 7) add a 120pF cap across the plates of the driver to supress oscillation. (good insurance if you don't have a scope) Feel free to comment...I'm always open to new ideas Regards; Carl Z Summit Amps http://www.geocities.com/summitamps

12/6/1999 3:21 AM

Carl Z

Re: "other" Fender mods: A little long winded

Bruce;

Lets see if I can tighten these explainations up a bit. First I was running under the assumption that woof was looking for more distortion in his rig. As I'm sure you're aware tone is mighty subjective so what's pleasing to one person may not be pleasing to another. Case in point...I've got a customer that loves his marshall biased like an ice cube. He loves the tone of his EL34's running at about 20mA/tube. To each his own.

Let's start with the input jack. Hot pickups such as humbuckers or even active pickups really could use some loading before they hit that first stage so you could reduce the size of that 1 meg resistor. The 68k grid stopper can be twiddled to give you a littl more high end and clarity by reducing the value to something like 33k or 22k. You could even remove it all together. Thumb rule here is that lower values will extend the high frequency response resulting in a crisper tone while larger values tend to dull the tone somewhat. Another tweak here is to run a bypass cap across it like some of the Ampegs. Trace (where'd ya go?) had a mod for this but I never got around to trying it.

Plate resistors: Without writing a book I'll try to clarify this a little. I'm ballparking the numbers here so i may be off a little. If we assume we're running with normal channel fender; gain -> tone -> recovery, changing the plate load on the second stage to 220k is going to give you an amplification factor somewhere in the neghborhood of 70 and bias current is going to be about 0.68mA. This gives you a bias voltage of just about 1 volt. I probably shouldn't have used the word headroom here, got sloppy with the terminology. What I meant to say was that the stage is going to run a little closer to pure class A and the THD is going to be much lower. I want to say it's about 1%, feel free to correct me if i'm wrong here. Unless you're running the amp flat out or again using hot pickups you probably won't drive it into distortion. With the volume maxed a strat will give you a little over a volt but by then you'll have power amp clipping. Hope I cleared that one up.

On to the tone stack....
I've tried the blues junior arrangement and it's not too shabby, just different. It's a quick easy mod to that can be undone in a couple of minutes if you don't like it.
From a purely engineering point of view passive tone controls really need to be placed after a low impedance source such as a CF stage. In this regard I've had some interesting results on a reverb amp by bootstrapping the first two stages and running the tone off the follower. Then tapping the tone control to send to the reverb circuit and back to the mixer stage. Definetly worth further investigation by the adventurous!

As for the grid resistor RC network I tend to prefer the 047 caps. 022 and 033 caps seem a little too constricted to me. There's that personal preference thing again.

On the feedback loop I like the sound of 22k/4.7k combination. To me it's a nice compromise. You could also bypass the 22k with a small cap of around 250p but I'm not a big fan of that. seems to roll of too many highs.

Anyway, to make a long story short here's what I like to do to my Fenders....
1) change the input grid stoppers to 33k
2) change the cathode bypass caps to 5uF.
3) change the power tube grid resistors to 100k and the grid stoppers to 4.7k
4) change the driver coupling caps to 0.047uF
5) change the driver input cap to 0.01uF
6) change the feedback loop values to 22k/4.7k
7) add a 120pF cap across the plates of the driver to supress oscillation. (good insurance if you don't have a scope)

Feel free to comment...I'm always open to new ideas

Regards;
Carl Z
Summit Amps
http://www.geocities.com/summitamps

12/6/1999 3:28 PM
Fred G.	I have now seen cutting the power tube grid resistors from 220k down to 100k or so suggested as a cure for Fender flatulence, eliminating parasitics, and now for improving or boosting gain a bit ? Can somebody give a little more info on the how's and why's of this topic, summarize, and/or generalize ? Thanks, Fred G.

12/6/1999 5:13 PM
Ron Orlando	Two questions: 1). On the Fender Standalone Reverb there is no 68kgridstop resistor coming from the input as we see in most fender amps. Has anyone stuck one in there and what did you get? 2). On the 5E7 schematic there is a 1.5k cathode resistor on the inverter. Have any of you guys hung a 25/25 cap along side of it to juice up the gain a tad??

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